The Principle of Complementarity Between International and Domestic Jurisdictions in the Prosecution of War Crimes

The article reconstructs the legal architecture of complementarity under the Rome Statute and articulates the ICC’s admissibility tests: unwillingness (shielding, unwarranted delay, lack of independence/impartiality), inability (total or substantial collapse/ unavailability of the national system), gravity, and the “same person/same conduct” standard. Drawing on Ongwen, Ruto and the Libya decisions (Saif Al-Islam Gaddafi; Al-Senussi), it distils how the Court distinguishes declaratory reforms from genuine investigative and prosecutorial action, assesses the availability of the accused, and evaluates a state’s capacity to secure a fair trial. A comparative institutional section demonstrates models from Nuremberg/Tokyo (exclusive jurisdiction over “major war criminals”), through ICTY/ICTR (tribunal primacy over national courts), to hybrid tribunals (SCSL, ECCC, Kosovo SC), situating the ICC’s contemporary role as complementing—rather than replacing—domestic justice. Focusing on Ukraine’s wartime context, the paper identifies admissibility risks (in absentia trials, use of pre-trial statements, special evidentiary rules) and proposes safeguards that demonstrate the genuineness and capacity of domestic proceedings (comprehensive audio-video recording, effective defence and cross-examination, reasoned judgments and proportionality of sanctions). It develops a practical algorithm for delineating jurisdictions and distributing criminal proceedings among national courts, the ICC, and possible hybrid mechanisms, taking into account the preconditions and triggers of jurisdiction, the admissibility tests (Article 17 of the Rome Statute), the “same person/same conduct” standard, and the “gravity” criterion. A roadmap for positive (proactive) complementarity is offered for Ukraine—capacity-building, harmonised evidence standards, structured cooperation with the ICC Office of the Prosecutor, and duplication-minimising protocols. The contribution lies in integrating the ICC’s admissibility doctrine with wartime procedural realities, clarifying terminology (primacy vs. “parallelism”), and introducing operational metrics of genuineness/capacity for oversight. The practical significance consists in developing policy tools for the prosecution of war crimes that are compatible with complementarity and fair-trial guarantees.